1. Field of the Invention
The present invention is directed to a pulse transformer for U-interfaces according to the echo compensation principle and to a method for the manufacture of a toroidal tape core contained in pulse transformers for U-interfaces.
2. Description of the Prior Art
In a digital telecommunication system, particularly an ISDN telecommunication system, the connection between a digital local exchange and the network termination ensues via a public two-wire line at whose ends what are referred to as U.sub.K0 or U.sub.HDSL reside. A schematic view of such an ISDN telecommunication system is shown in FIG. 1. Such an ISDN telecommunication system is composed of a network termination (NTBA1), of an ISDN local exchange 2, a U.sub.K0 transformer 3, a current-compensated double inductor 4, an integrated circuit 5 that, for example, can be an IEC PEB 2090 as well as of a power supply 6 with a DC/DC converter.
Dependent on coding and pulse repetition rate f.sub.0, one distinguishes between the 4B3T system and the 2B1Q system at the U.sub.K0 interface. The 4B3T system has a pulse repetition rate f.sub.0 of 60 kHz and a coding of 4 bits (on 3 ternary symbols). The 2B1Q system, by contrast, exhibits a pulse repetition rate f.sub.0 of 40 kHz and a coding of 2 bits onto a quaternary status. These differences have effects on the spectral distribution of the magnetization current of the U.sub.K0 transformer. This influence on the spectral distribution of the magnetization current can be derived from FIG. 2. As can be seen from FIG. 2, there is a big part of low-frequency components with high current amplitude in the spectrum of the 2B1Q code, by contrast whereto higher-frequency components dominate given 4B3T code. In the case of a U.sub.K0 -4B3T interface, the line length between the transformers can amount to up to 5 km. Given the U.sub.K0 -2B1Q interface and the U.sub.HDSL interface, the line length between the transformers is usually somewhat less. The error rate of the transmitted cos.sup.2 -shaped pulse must always be below 10.sup.-7.
Added thereto as a complicating factor is that direct currents for remote feed of the terminal subscribers can be superimposed on the information pulses, these amounting to up to 80 mA according to the system under consideration (4B3T, 2B1Q) and even more given HDSL. The supply currents lead to a dc bias H.sub.dc of the interface transformer magnetic core whose height is critically determined by the embodiment of the transformer magnetic core and the magnetic core material employed. It is thereby a fundamental condition that the message-oriented characteristics defined in the applicable standards such as pulse and impedance masks must be adhered to even under the maximally possible or, respectively, under changing dc bias.
The following demands are made of the interface transformers utilized at both sides of the link:
d) minimal structural volume, PA1 e) suitability for the transmission code systems, PA1 f) main inductances 1 through 80 mH dependent on the wired integrated circuit, PA1 g) dc superimposition of the remote feed, PA1 h) transmission range&gt;800 km (given 4B3T) or, respectively, 15 loops according to ANSI (given 2B1Q), PA1 i) low core weight and SMD (surface mounted device) capability.
4B3T-U.sub.K0 (see FZT 1 TR 220 and ETSI Standard ETR 80; 60 kHz, 120 kBaud) PA2 2B1Q-U.sub.K0 (see ANSI-T1.601 and ETSI Standard ETR 80; 40 kHz, 80 baud) PA2 2B1Q and CAP-HDSL (see ETSI Standard ETR 152; 2 Mbit/sec), PA2 0 through 60 mA given 4B3T interface transformers PA2 0 through 80 mA given 2B1Q interface transformers (at liberty according to attraction standard)
Up to now, ferrite have been employed as material for the transformer cores, ferrites of the ferrite materials N27 and N28 having been particularly utilized. The required dc biasability was thereby achieved by means of shearing with slots of the ferrite ring cores. As a result of this great shearing of the magnetic circuit. The originally non-linear BH loop of the ferrite material is so highly linearized that the transformer satisfies the required range or thereby, bit error rate. The disadvantage is that the shearing causes a decrease of the effectively active permeability down to values around 200. In order to achieve the required main inductance of 1 through 80 mH, however, the volume must be designed very large as a result of the design given ferrite ring cores.
It has also proven a disadvantage given the employment of ferrite ring cores that high numbers of turns of the winding of the primary side and secondary side are required that can lead to ohmic losses and capacitatively cause noise effects.
Further, European Application 0 378 823 discloses an interface transformer for an S.sub.0 interface that comprises a magnetic core of an amorphous cobalt-based alloy with a permeability between 25 000 and 95 000. The anisotropy energies of the alloys known from European Application 0 378 823 lie in the area of 5J/m.sup.3.
The magnetic cores disclosed by European Application 0 378 823 are only suited for interfaces with a dc bias in the area of 10 mA and a transmission range of up to 150 m.